Various principles and methods are known in the field of electronic distance measurement. One approach is to emit frequency-modulated electromagnetic radiation as an emission signal toward the target to be measured and subsequently to receive at least a part of the radiation returning from the target as a reception signal, also called an echo or echo signal. The target to be measured can comprise in this case both reflective backscattering characteristic, for example, if retroreflectors are used, and also diffuse backscattering characteristic.
After the reception, the echo signal is overlaid with a local oscillator signal to generate a beat signal, wherein the beat signal has a beat frequency correlating with the time-of-flight of the emission signal. The distance to the target may then be derived on the basis of the beat frequency.
The mixing/overlay is primarily used to transform the reception signal toward lower frequencies and amplify it, whereby the time-of-flight and thus—with known propagation speed of the radiation used—the distance to the target to be measured can be determined.
So-called FMCW distance meters (FMCW: “frequency-modulated continuous wave” radar), also called frequency-modulated continuous wave radar devices or FMCW radar devices, enable an absolute distance measurement to a target to be measured.
A tunable laser source is used in an FMCW arrangement. In the embodiment which is simplest in principle, in this case the tuning of the optical frequency of the laser source is performed linearly and at a known tuning rate, wherein the absolute wavelength of the emission signal thus generated is only known up to a certain degree, however. The reception signal is overlaid with a second signal, which is derived from the emitted emission signal. The resulting beat frequency of the mixed product, the interferogram, is a measure of the distance to the target.
Diverse refinements of this fundamental embodiment are known in the prior art, for example a use of a reference interferometer to measure the tuning behavior of the laser.
The distance measuring devices employed for implementing these methods typically use a signal generator, by means of which a signal, for example, a rising or falling frequency ramp, is applied to a radiation source which can be modulated. Lasers which can be modulated are typically used as radiation sources in the optical field. Emission and reception optical units are used for emission and reception in the optical field, from which a detector or quadrature detector for heterodyne mixing, A/D converter, and digital signal processor are connected downstream.
The change of the frequency of the emitted emission signal represents the scale of the measurement. Depending on the accuracy requirement for the distance measurement, this scale can be verified or determined more accurately by means of an additional measurement. Adequately linear tuning of the laser source often requires additional effort, for example. For this purpose, for example, a part of the emitted radiation is guided via a reference interferometer having defined reference length. The frequency change over time of the emitted emission signal can be concluded from the resulting beat product on the basis of the known reference length. If the reference length is unknown or unstable, for example because of temperature influences, it can thus be determined via an additional calibration unit, for example, a gas cell or a Fabry-Perot element.
In the most favorable case, the target is a target resting in relation to the distance meter, i.e., a target which has an unchanging distance over time in relation to the distance meter. However, absolute distance measurements can also be carried out on moving or vibrating targets with suitable compensation measures.
A radial movement of the target in relation to the distance meter results in a Doppler shift of the reception frequency because of the Doppler effect. The Doppler shift can be compensated for, however, by a combined measurement by means of successive rising and falling frequency ramps, for example, since the Doppler shift is equal for both ramps in the case of a constant radial velocity of the target, wherein the beat frequencies generated by the two ramps have different signs, however.
The usable measurement rate is also halved by the use of chronologically successive opposing ramps, however, i.e., with successive variations of the chirp sign. Moreover, this approach is based on a constant relative target velocity being provided during the cycle time for the two ramps. This assumption of a constant relative velocity of the target in relation to the distance meter is often inaccurate in practice, however, wherein accelerations and/or vibrations of the target during the measuring procedure, speckle effects, or other effects result in non-negligible measurement fluctuations in the distance measurement.
To remedy these problems, for example, two simultaneous and opposing frequency ramps are used in the prior art, i.e., wherein radiation is emitted having two radiation components, wherein the frequency of a first radiation component is tuned “upward”, i.e., toward higher frequencies, and simultaneously the frequency of a second radiation component is tuned “downward”, i.e. toward lower frequencies. The requirement for a constant relative radial velocity of the target is thus limited to a short time window. Moreover, for example, a reduction of the measurement rate is also avoided by such so-called opposing chirps.
To be able to metrologically separate the radiation components, various measures are known in the prior art, for example, polarization-based, spectral-based, or algorithmic separations.